However, one factor that has yet to be properly determined is the best site for stem cell injection. Previous work by scientists at the Keio University School of Medicine in Japan has shown that injection of neural stem cells and neural progenitor cells (NS/PCs) into non-injured sites by either intravenous or intrathecal (introduced directly into the space under the arachnoid membrane of the brain or spinal cord) administration failed to produce sufficient engraftment of stem cells at the site of injury.

Instead cells were trapped in the lungs and kidneys, and many mice even developed fatal lung conditions as a result of intravenous administration (see Takahashi Y., et al., Cell Transplant. 2011;20(5):727-39). These data convinced them that intralesional application of the stem cells (injections directly into the damaged site of the spinal cord) might be the most effective and reliable method for NS/PC tranplantations.

A new study by the Keio group has attempted to ascertain the efficacy of the intralesional injections. Mice with spinal cord injuries were injected with NS/PCs that had been derived from mice that expression glowing proteins. This allowed the injected cells to be tracked with bio-luminescence imaging (BLI).

The principal investigator of this research is Masaya Nakamura from the Department of Orthopedic Surgery at the Keio University School of Medicine. Dr. Nakamura and his team gave mice spinal contusions at the level of the tenth thoracic vertebra. Then some mice were given low doses and others high doses of NS/PCs that were derived from fetal mice (for those who are interested, low dose – 250,000 cells per mouse; high dose – 1 million cells per mouse) nine days after spinal cord injury. These mice were further divided into two groups: those injected at the lesion epicenter (E), those injected at sites at the front and back of the lesion (RC for “rostral/caudal”). Thus there were four groups total: High dose E, High dose RC, Low dose E, and Low dose RC.

All four groups showed better functional recovery than the control group, which was injected with phosphate buffered saline. BLI showed that the number of cells that survived in each of the four cell-transplanted groups was about the same across these groups. Thus injecting more cells does not lead to greater numbers of surviving neural stem cells. This makes sense, since the damaged spinal cord in very inhospitable place for transplanted cells.

However, when the mice were examined for the expression of particular brain-derived neurotropic factors, the expression of such genes was higher in the RC-injected mice than in the E-injected mice. These results seems to explain why the transplanted NS/PCs differentiated more readily into neurons in the RC-injected mice rather than a type of glial cell known as an astrocyte, as was the case in the E-injected mice.

Human Astrocytes

Nakamura and his team interpreted these results to mean that the environments of the E and RC sites can both support the survival of transplanted NS/PCs during the sub-acute phase of spinal cord injury. The authors conclude with a practical note: “Therefore, we conclude that it is optimal to graft a certain threshold number of NS/PCs into the epicenter lesion during the sub-acute phase of SCI, and thereby avoid causing further iatrogenic injury to the intact RC regions of the spinal cord.”

Hopefully Nakamura’s work will be translated into further human clinical trials. One feature of this study is that a particular threshold of stem cells survive when injected into the spinal cord and injecting larger numbers of cells does not increase the number of surviving cells. Injecting more cells might only contribute to the cell debris in the spinal cord. This is certainly a good thing to know when conducting clinical trials with neural stem cells in spinal cord-injured patients.